Determining the frequency of an optical signal is desirable in a variety of measurement environments. For example, determining the frequency of an optical signal applied to a measurement receiver enables the frequency scale of the measurement receiver to be calibrated, whereas determining the frequency of an optical signal from an optical source enables the frequency tuning characteristics of the optical source to be calibrated.
Absorption cells are often used when determining the frequency of an optical signal. These absorption cells are typically gas cells that contain one or more gases, such as acetylene and methane, or hydrogen cyanide, and provide absorption lines at optical frequencies that are spaced over a broad frequency range. The absorption lines are stable over time and stable over a variety of environmental conditions, making the absorption cells well-suited frequency standards in optical frequency determinations.
U.S. Pat. No. 6,249,343 B1 to Wang et al. discloses an absorption cell configured to frequency calibrate a measurement receiver, such as an optical spectrum analyzer. Alternatively, absorption cells can be configured to frequency calibrate optical sources, such as a tuneable laser source, as shown in FIG. 1. In FIG. 1, the absorption cell is illuminated by the tuneable laser source while a broadband receiver, such as an optical network analyzer or power meter, detects a resultant optical signal at the output of the absorption cell. Since optical energy is absorbed at frequencies precisely defined by the absorption lines of the absorption cell, the frequency of the tuneable laser source can be accurately determined at those frequencies by observing the position of amplitude notches or minima detected in the frequency response measured by the receiver. While determining optical frequency at various frequency positionsxe2x80x94including those between the absorption lines, is critical for characterizing optical sources that have nonlinear tuning characteristics, or for optical signals in dispersive media, the prior art configuration shown in FIG. 1 does not readily enable accurate frequency determinations to be made at frequency positions other than those of the absorption lines. Accordingly, there is a need for a frequency discriminator that enables the frequency of an applied optical signal to be determined not only at the absorption lines of an absorption cell, but also at frequency positions between absorption lines.
An optical frequency discriminator constructed according to the embodiments of the present invention includes an interferometer cascaded with an absorption cell. In response to an applied optical signal, the cascaded arrangement provides a composite signal that is a superimposition of a cyclical fringe signal provided by the interferometer and a series of absorption lines provided by the absorption cell. A receiver samples the composite signal and maps sample positions of the acquired samples and corresponding optical frequencies of the optical signal, based on the cycles of the fringe signal and identified frequencies of predesignated absorption lines in the series. Alternative embodiments of the present invention are directed toward an optical frequency discrimination method.